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Topic: Redox Lab Help  (Read 3646 times)

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Offline Undergradstudent

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Redox Lab Help
« on: September 21, 2012, 12:00:03 AM »
I have a few lab questions..could someone check them for me? :(

1. 1-Benzylnicotinamide chloride can be prepared by heating nicotinamide with benzyl chloride in DMSO. Give a mechanism for the reaction and explain why alkylation occurs at the pyridine nitrogen. Why is DMSO a good solvent for this reaction?

Ans. Is this just a simple Sn2 reaction?
alkylation occurs at the pyridine nitrogen because amides are weaker nucleophiles
DMSO is dipolar aprotic solvent so nucleophile stays anionized but can be stabilized by polarity

2. 1-Benzylnicotinamide is similar to a naturally occuring enzyme cofactor that is used reduction/oxidation reactions. What is the name and structure?

Ans. NAD+/NAD(P)H??

3. Ethyl benzoylformate is not reduced by 1-benzyldihydronicotinamide alone. In the presence of magnesium chloride, however, racemic ethyl mandelate is formed in quantitative yield. Explain with a mechanism how MgCl2 accelerates the reaction.

Ans. MgCl2 could help stabilize the negative charge on the O on the carbonyl group being reduced?

4. When ethyl benzoylformate is treated with (R)-N-(alpha-methylbenzyl)- dihydronicotinamide in the presence of an equimolar amount of magnesium chloride in aqueous acetonitrile solution, the ethyl mandelate produced is 60% R and 40% S.

a.   Draw formulae indicating the structure and configuration of (R)-N-(alpha-methylbenzyl)- dihydronicotinamide and of the two enantiomers of ethyl mandelate.

Ans. http://www.curvedarrow.com/chem/mandelate.jpg
are these correct?
How do we know which N the substituent is on? (for (R)-N-(alpha-methylbenzyl)- dihydronicotinamide)

b.   Explain why the product is not a racemic mixture (you do not have to explain which enantiomer is favoured).

Ans. sterics?

c.    What is the enantiomeric excess of the ethyl mandelate?

Ans. I don't even understand this question :(
Could someone rephrase it for me?

Offline Babcock_Hall

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Re: Redox Lab Help
« Reply #1 on: September 21, 2012, 10:08:34 AM »
With respect to problem 2, I would slightly rephrase.  NAD and NADP are the oxidized forms, not NADH and NADPH, which are the reduced forms.  With respect to problem 1, I would note that the nucleophile is neutral and will develop a partial positive charge in the t.s., and the l.g. will develop a partial negative charge.

Offline Undergradstudent

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Re: Redox Lab Help
« Reply #2 on: September 21, 2012, 01:11:38 PM »
Thanks.

Could you give me a clue for 4b? Also what would be used to protonate the negative charge on carbonyl which MgCl2 stabilizes in question 3 and 4? Would it be the solvent in question 4 (aqueous acetonitrile solution)?

Offline Babcock_Hall

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Re: Redox Lab Help
« Reply #3 on: September 21, 2012, 04:38:49 PM »
It could be the water that is the source of the proton.  Let me try a general explanation out, and we can make it more specific as need be.  Two transition states that are enantiomers of each other have equal energy; therefore, the rate constants of the reactions will be equal.  Two transition states that are diastereomers of each other do not in general have equal energy and need not have rate constants that are equal.

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